1. Field of the Invention
This invention relates to a method of regenerating absorbent. More specifically, this invention relates to a method of regenerating an absorbent to be employed in a reformer filled with a reforming catalyst and an absorbent containing lithium silicate for generating hydrogen, thereby enabling the absorbent to recovery its absorption properties.
2. Description of the Related Art
Since hydrogen (H2) to be generally employed as the fuel for a fuel cell is not abundant in nature, it is generally produced by means of steam reforming method wherein a fossil fuel such as methane or kerosene is employed as a raw material and reacted with steam (H2O) heated to high temperatures in the presence of a reforming catalyst. In a case where methane (CH4) which is a major component of natural gas and town gas is employed as a feedstock gas for example, this steam reforming method can be represented by the following reaction formula:CH4+2H2O4H2+CO2  (1)
Recently, there has been also studied to produce hydrogen from a raw material constituted by ethanol (C2H5OH) which is now attracting much attention as a renewable energy source that can be produced from plants (see F. Frusteri et al., Journal of Power Sources, 132, 139 [2004]). The steam reforming reaction wherein ethanol is employed as a feedstock can be represented by the following reaction formula:C2H5OH+3H2O6H2+2CO2  (2)
These reactions however are deficient in that a large number of by-products are generated, so that the production gas contains such impurities as methane, carbon monoxide (CO), and carbon dioxide (CO2). In particular, the production gas present immediately after the steam reforming reaction contains carbon monoxide at a volume ratio ranging from several percent to several tens of percent, the exact ratio depending on the reaction conditions and the raw materials. For this reason, a carbon monoxide (CO) converter is generally connected to the downstream end of the reactor (see “The Front of Hydrogen Energy” (2003), page 36, Industry Research Group). The production gas that has been treated by means of this CO converter contains carbon monoxide which has been reduced in concentration down to 0.5% by volume and is subsequently transferred to a gas-refining process.
JP-A 10-152302 (KOKAI) and JP-A 2002-274809 (KOKAI) disclose, respectively, a method for efficiently producing hydrogen, wherein lithium composite oxide acting as an inorganic absorbent is employed in addition to the conventional reforming catalyst in the steam reforming reaction accompanying the generation of CO2 as a by-product, thereby making it possible to remove CO2 from a high-temperature reaction cite heated above 400° C. and to shift the chemical equilibrium thereof toward the main product-generating side. Among the lithium containing oxides, lithium silicate is most suited to shifting the chemical equilibrium in this manner since it is capable of absorbing CO2 especially at a high rate. The absorption of CO2 by lithium silicate can be represented by the following reaction formula:Li4SiO4+CO2Li2CO3+Li2SiO3  (3)
In reaction formula (3), when the reaction in the rightward direction takes place, CO2 is allowed to react with and absorbed by lithium silicate. As set forth by M. Kato et al., in Journal of Ceramics Society of Japan, 113(3), 252 (2005); by Essaki et al., in the Proceedings of 15th Meeting of the Japan Institute of Energy (2006); and by Suzuki et al., in the Proceedings of 37th Autumn Meeting of Society of Chemical Engineers, Japan (2005), it has been confirmed experimentally that, in the cases of methane and ethanol, it is possible to shift the equilibrium of the reaction with high-temperature steam and so promote the generation of hydrogen while reducing the concentration of carbon monoxide by-product.
However, experiments by the present inventors investigate that when an absorbent consisting of lithium silicate co-exists with a reforming catalyst in the steam-reforming reaction cite of ethanol, carbon is deposited on the surface of the absorbent. The carbon of this kind is more likely to be produced as the number of carbon atoms per molecule in the feedstock gas increases, so that, in the case of a compound such as ethanol containing two carbon atoms or propane containing three carbon atoms, the deposition of carbon would become more prominent. Further, the quantity of deposition of carbon can be increased as the repeated use of the absorbent where CO2 is enabled to be released to regenerate the absorbent. Since deposited carbon would cover the surface of absorbent to inhibit the reaction thereof, the absorption performance of the absorbent would be degraded. Therefore, in the regeneration of the absorbent, it is necessary to release CO2 and, at the same time, to remove the carbon thus deposited from the absorbent.
As for the method for removing the deposited carbon, it is conceivable to employ, for example, a method wherein steam or air which is capable of reacting with carbon is passed through the absorbent. This method however is accompanied with various problems that, in the case of steam, it requires a lot of energy in order to vaporize water and, in the case of air, a high-temperature region is formed locally in the absorbent on oxidizing the carbon, thereby resulting in the sintering of catalyst and absorbent and hence resulting in the degradation in performance of absorbent.
Meanwhile, a method for continuously producing hydrogen is disclosed by K. Essaki et al. in Proceedings of 16th World Hydrogen Energy Conference (2006), wherein a plurality of reactors, each provided with an absorbent and filled with a reforming catalyst, are installed in a hydrogen-producing apparatus. In the case of the absorbent comprising lithium silicate, the release of CO2 can be accomplished within 30 minutes at a temperature of 650° C. in a nitrogen atmosphere, so that a feedstock gas for reforming is permitted to flow in the same direction as the flow of nitrogen on regenerating the absorbent. Further, since it becomes more advantageous as the concentration of CO2 in an atmosphere becomes lower with respect to the reaction equilibrium on regenerating the absorbent, it has been considered more advantageous to employ a method wherein nitrogen gas is passed through the absorbent. However, according to this document, since the feedstock gas employed therein is methane and hence the aforementioned deposition of carbon can hardly take place, the conditions for the regeneration are set, taking only the release of CO2 into consideration.
According to Takenaka, Lectures by New Energy Section of Petroleum Society, pp. 58 (2006), there is a description with respect to the elimination of carbon that has been precipitated in a ceramic porous body having almost the same configuration as that of the absorbent, wherein the carbon is heated at a temperature of 650° C. in an atmosphere containing 50% by volume of carbon dioxide. The elimination of carbon according to this method can be performed according to the following reaction formula:C+CO22CO  (4)
Due to the equilibrium, this reaction is enabled to work more advantageously in the elimination of carbon as the concentration of CO2 in the atmosphere becomes higher. However, when the aforementioned elimination of CO2 from the absorbent is taken into account, this reaction contradicts to the CO2-eliminating conditions. It has been confirmed by the present inventors from the experiments wherein an absorbent consisting of lithium silicate was heated at a temperature of 650° C. in an atmosphere containing 50% by volume of carbon dioxide that it was almost impossible to release CO2 and hence the regeneration of the absorbent could hardly be achieved.